Draco and the imperius

[Jo Marelvy Samudio]

No: HPFGUIDX 137487

In the tower scene before the DE's arrive, DD asks 
Draco "...Rosmerta. How long has she been under the Imperius Curse?"

Malfoy did imperiused Madam Rosmerta, right? At least since Katie was 
given the necklace, so how come there were no owls with warning about 
underage use of magic? Does this mean that Malfoy had already turned
17 by the time he performed the Imperius on Rosmerta? We do know that 
he is taking the apparition course, so he must be turning 17 by the
end 
of August.
If he had not yet turned 17, and an owl with the undarage use of
magic 
warning was to be sent, where would it arrive? Hogsmade? Hogwarts?
We now know that although the MM cannot possibly know who in fact 
performed the underage magic, they anyway know that some was
performed 
and DO send an owl.

Any ideas? Does anyone happen to know when Draco's birthday is?
Jomarelvy